The invention relates to a device for the variable braking of traveling threads, wires or the like, particularly for use in connection with the introduction of the filling thread in looms, and to a method for the operating of such a thread brake.
A thread brake is known from French Patent Application 23 75 366. In that case, the thread is passed between two brake parts which can be brought under spring action towards each other. One of the two brake parts is stationary and not under spring action, while the other brake part is under spring action and is movable with respect to the stationary brake part so that the braking of the thread can be varied by a change in the intensity of the tension of the spring obtained as a result of its movement. The spring-actuated brake part can also be spaced away from the stationary brake part by a push rod, as the result of a cam rocker drive, so that unbraked travel of the thread can be obtained. Such a thread brake has the disadvantage that the force of the spring must be relatively great in order effectively to brake the thread. On the other hand, in order to obtain free travel of the thread, the spring-actuated brake part must be lifted completely away from the stationary brake part. In this connection, the spring of the spring-actuated brake part must be at least partially relaxed. This results in relatively large displacement paths of the push rod which drives the spring-actuated element. Furthermore, limits are set for such an arrangement with regard to the cycle time between the braking and release of the thread since the relaxation of the spring can no longer follow along with high cyclic frequencies.
From European Patent Application 0 384 502 a thread brake is furthermore known in which a rotatable, non-spring-actuated brake part acts on a prestressed spring blade. With this arrangement, however, no cyclic change of the braking of the thread is provided. The rotating cylindrical body which forms the non-spring-actuated brake part serves merely to remove, by rubbing, particles of dust present on the thread.
Another example for the invention was the device for the differential braking of traveling threads proposed in Utility Model 91 13 430 (corresponding to U.S. Pat. No. 5,305,966 issued Apr. 26, 1994) which does not constitute a prior publication or prior art. The device described therein already has a stationary leaf spring which is acted on by a cylindrical brake part. This brake part is driven by an electric motor and has a cylindrical first section the circumferential surface of which holds the spring under pretension in the position of release for the travel of the thread. A second partial section of the body has a circumferential section which is reduced in cross section. The thread passes between this second partial section and the leaf spring. If the cylindrical body is aligned with the window towards the leaf spring, the thread is unbraked. After a further rotation of the rotatable body by a given angular amount, the thread is acted on by the circumferential section of unreduced cross section of the second partial sections so that the thread-braking position is reached, the thread being then clamped between the leaf spring and the cylinder surface. This device operates in fixed-cycle operation, the drive motor traveling back and forth between two stops. In order to obtain a high cyclic frequency, the motor must be strongly accelerated as well as strongly decelerated. The use for instance of mechanical stops for the braking of the motor is not optimal.